Method and apparatus for aircraft control of airport landking lights



Jan. 7, 1964 L.. F. LEMM ETAL 3,17299 MRTROD AND APPARATUS POR AIRCRAFTCONTROL OF AIRPORT LANDING LIGHTS Filed oct. 28, 195e 2 sheets-sheet Ifgndf l (To F5959 Lew-se 1:7 .LE/14M, ZI/Rey WE-ESON,

INVENTORS.

Jan. 7, 1964 METHOD AND APPARATUS FOR AIRCRAFT CONTROL OF' Filed Oct,28, 1958 L F. LEMM ETAL AIRPORT LANDING LIGHTS 2 Sheets-Sheet 2 www@@MMA United States Patent O 3,117,299 METHOD AND APPARATUS FR AIRCRAFTCONTRL F AIRPORT LANDING LIGHTS Lester F. Lemm, 8180 E. 21st,Westminster, Calif., and

Harry W. Beeson, Fullerton, Calif., assignors, by direct and mesneassignments, of six-twelfths to Charles A.

Bauer, Huntington Beach, Calif., three-twelfths to said Lester F. Lemm,and three-twelftlis to Robert P. Swift Filed Oct. 28, 1958, Ser. No.'770,062 Claims. (Cl. 340-26) The present invention relate-s to a methodand apparatus by which the landing lights of an airport may be turned onunder control of the pilot of an aircraft which is about to make a nightlanding.

Many small airports are used only occasionally and hence are manned byoperators only during certain restricted hours. If an operator -were tobe maintained on duty throughout the night the cost of his wages wouldprohibitively increase the operating expenses of the air ort.

P Two different arrangements are widely used by small airports which`are unmanned during the night. According to one arrangement the landinglights are permitted to burn continuously during the night so that anycraft which seeks -to land may do so. Modern landing lights have beendeveloped which have a very high degree of visibility during fog andsimilar atmospheric conditions. Such lights require a large amount ofelectricity for their operation, however, hence the small air-port whichkeeps its landing lights burning during the night mus-t incur a verysubstantial expense for that purpose.

The other arrangement used lby unmanned airports is to keep the landinglights turned off, and to utilize fthe services of -a policeman, nightwatchman or other available person for turning them -on when and asneeded. According to this arrangement the aircraft is required to buzzthe field in order to indicate a desire to land. The lights are thenturned on by the policeman, night watchman or other responsible person,although it may be necessary f r lhim to drive several miles by-automobile in order to reach the airport. The limitations of such anarrangement are readily apparent.

According to the present invention a method and apparatus are providedwhich permit the landing lights to be turned on under the control of thepilot who is about to land. A predetermined set of signals istransmitted from the aircraft in a predetermined frequency channel. Aradio receiver and a light control unit are provided at the airport, thereceiver being tuned to the predetermined transmission frequency -andbeing energized in a receptive condition at all times. 'I'he lightcontrol unit is coupled to the receiver and is adapted to respond to thepredetermined set of signals for turning on the landing lights.

In carrying out the method of the present invention the radiotransmitter in the aircraft cannot feasibly be directionalized, hencethe problem arises that where two or more airports are located in thesame vicinity there must be some means of selecting the particular oneat which the pilot Wishes to land. In -accordance with the invention,therefore, a code is provided such that a different identification isassigned to each airport in a particular geographical area. Intransmitting the command for the lights to be turned on the pilotselects the code identications for the particular airport at which hewishes to land.

An object of the invention, therefore, is to provide a method andapparatus for turning on the landing lights of 3,ll7,29l-l Patented Jan.7, 1964 ice an airport under the control of the pilot of an aircraftwhich is about to make a night landing.

Another object of the invention is to provide a method and apparatuswhereby the pilot of an aircraft about to make a night landing mayselect a particular landing eld, and may cause its landing lights to beturned on.

A further object of the invention is to reduce the maintenance expenseof airports by providing automatic means for turning the landing lightson and off at night, thus making `it unnecessary for the airport to bemanned during the night.

An additional object of the inven-tion is to provide a light controlapparatus which may be installed in an airport and utilized inconjunction with conventional radio communication equipment for turningon landing lights when an aircraft approaches for a night landing.

The above and other objects of the invention will be more readilyunderstood from the following description in conjunction with theaccompanying drawing in which:

FIGURE l illustrates an aircraft which is about to make a night landing;

FIGURE 2a illustrates in block form the radio communication system whichis utilized to carry out the novel method of the invention;

FIGURE 2b is a schematic wiring diagram of a circuit for transmittingpulses from the receiver to the light control unit of FIGURE 2a;

FIGURE 3 is a schematic diagram, partially in block form, of a counterand selection circuit for identifying the received pulse series;

FIGURE 4 is a schematic wiring diagram of the first tive stages of thecounter of FIGURE 3;

FIGURE 5 illustrates schematically the timing mechanism which isinitiated into operation by a signal from the circuit of FIGURE 3 forturning on the lights.

Referring now to the drawings, FIGURE 1 illustrates an aircraft A whichis about to make a night landing and which was faced with a choice as towhether to land ata landing field -B or a landing eld C. 'I'he pilothav-ing already decided to land at B and having transmitted thecorresponding set of coded signals in accordance with the invention, thelanding lights of B have been turned on and clearly illuminate thelanding strip.

The method of [control provided by the invention has many -verypractical advantages which will be described in conjunction with FIGURES2a and 2b. The code system selected in accordance with the invention issimply a series of pulses, the number of pulses in the series differingfor each landing field in a particular area. Thus, for one landing fieldthe code signal Will be four pulses while for other landing fields inthe same area the code will be tive, six, seven or eight pulses.

As shown in FIGURE 2a the signals transmitted by transmitter D on theaircraft are received by a radio receiver E at the landing field fromwhence they are fed to the light control unit F. Receiver E is turned toa single fixed frequency with the tuning being preferably maintained bya crystal. The tixed frequency preferably lies in the VHF range and may,for example, be 122.8 meigacycles. Transmitter D may be a conventionalvoice transmitter which is adapted to operate on any of several selectedfrequencies. Transmitter D is preferably equipped with a crystal whichis tuned to the receiver frequency (here assumed to be 122.8 rnc.) andis also provided with a selector switch by which the pilot or radiooperator may select a desired transmission frequency.

In accordance with the invention the pilot of the aircraft rn-ay turn onthe landing lights simply by setting the selector switch to thepredetermined transmission frequency (such as 122.8 mo), and thenpressing his microphone button the appropriate number of times. Eachtime the microphone button is pressed the transmitter is quency to whichthe receiver is tuned. It is then necessary at the receiver to detectonly the beginning of each reception of the carrier wave.

In other words, each time the pilot depresses his microphone button thecarrier wave is turned on until he releases the button. For the purposesof the present invention it is largely inmaterial how long the carrieris turned on since there is no necessity for distinguishing between dotsand dashes It is also largely immaterial as to the magnitude or strengthof the carrier wave which is received at the landing iield, so lon-g asit exceeds the necessary minimum for reliable communication.

Referring now to FIGURE 2b it will be seen that in the receiver E arelay coil 90 `is provided which is coupled in parallel between theautomatic volume control (AVC) line 91 and ground. Normally open relaycontacts 92 are controlled by coil 90, Contact 92a being connec-ted toground while 9211 is connected via a lead -93 to the light control unitF. A resistor 94 and a capacitor 95 are coupled in parallel With relaycontacts 92 to permit sensitive operation Witho-ut sticking. I

In light control unit F a relay coil 97 is coupled between a source ofpositive potential E+ and the signal line 93. A normally open pair ofrelay contacts 98 are controlled by the coil 97 for intermittentlyapplying the positive B-ipotential Ito a pulse input line 20.

The operation of the circuit of FIGURE 2b is as follows. Whenever thecarrier is turned on the AVC voltage in the receiver increases in astep-wise manner and causes relay contacts 92 to close. Current thenflows through `coil 97 causing contacts 98 to close and a positive pulseto be applied to the counter circuit. When contacts 92 open and permitcontacts 98 to open, a conresponding negative pulse appears on the inputline 20, but the counter circuit included in light control unit F isinsensitive to negative pulses as will appear from the ensuingdescription.

The electronic circuitry illustrated in FIGURE 3, 4 and 5 performs thefunction of receiving and identifying the pulse series and of turning onthe lights in response thereto. One special feature of the circuitoperation is that the requisite number of pulses must be received withina speciiied time. For example, if the equipment has been set to acceptonly a series of iive pulses then it is necessary that live separatelyidentied pulses be received within a fixed, relatively short timeinterval, in order to turn on the lights.

Another unique feature of the circuit operation is that if more than thedesired number of pulses are received within the speciiied time intervalthe lights will not be turned on. Thus, if the equipment is set toaccept five pulses in a /z-second interval but six pulses are receivedwithin this period of time then the lights will not turn on.

An additional feature of the circuit operation is that the timingmechanism may be reset at any tme to start from the beginning of thecycle. Thus, it the pilot ot an aircraft attempting to make a land-ingis not certain whether the lights will remain on long enough to completehis landing he may at any time send a new series of pulses which willcause the timing mechanism to reset and hence cause the lights to remainon for a predetermined period of time after the reception of the lastset of turn-on signals.

Referring now to FIGURE 3 it will be seen that count pulses indicated byan `arrow 20 are applied to a counter circuit 21 illustrated only inblock form. The counter circuit is a 9-stage ring counter of the type inwhich each applied count pulse causes one additional counter stage tobecome conductive. That is, after tWo count pulses have been receivedonly the first two stages are conductive, whereas after a total of sevencount pulses have been received all of the first seven stages areconductive. Wiring details of a suitable counter circuit are illustratedin FIGURE 4 and will be discussed in a later paragraph.

In FIGURE 3 the counter Z1 is shown to provide output signals l, S4, S5S9 which are produced on corresponding output leads. Each output signalis derived `from the corresponding stage of the counter. Lead l providesa positive output voltage when its corresponding coun-ter stage (thefirst stage) is non-conducting, and becomes isolated from the circuitwhen the lfirst stage conducts. The other output leads S4 Sg providepositive potential when their corresponding stages are conductive.

An electronic timer is provided which includes acapacitor 25, resistors26 of which -a particular one is selected by the switch 27, and aninductance coil 28. Inductance coil 28 torms a part of a relay RLm aswill subsequently be explained. The capacitor, resistor and inductancecoil of the electronic timer are arranged in a series loopconfiguration. Prior to the time when Vany count pulses have beenreceived, the rst'stage of counter 21 remains non-conductive and outputlead 'S1 provides a positive potential to the electronic timer. Thispotential is applied across capacitor 25 which is therefore normallycharged, and also across the resistor and inductance coil in series sothat a lfirst predetermined magnitude of current normally flows throughthe inductance coil 28. When the -rst count pulse -is received and theiirst counter stage becomes conductive the positive potential 'Sl is nolonger supplied to the electronic timer, and energy previously stored incapacitor 25 then flows through resistor 26 and induct-ance coil 28.

Counter 21 is energized via a lead 30 connected to B+ and a .groundreturn lead 31. The contacts of relay RLm are connected in series in theground return line 31 for the purpose of selectively disabling thecounter when the electronic timer has completed its timing cycle. Thus,the contacts of relay Riem are normally held closed by the currentflowing through inductance coil 28, and when the timing cycle starts andcapacitor 25 gradually loses Aits charge the current through inductancecoil 28 eventually drops to a second predetermined magnitude which isinsufficient to keep the relay closed. The relay then opens and disablesthe counter circuit and inhibits any of the output signals S4 S9 frombeing produced. The duration of the timing cycle of the electronic timeris determined by the potential to which capacitor 25 is normally chargedas well as by the R, L and C of the values of the circuit. Switch 27permits the selection of any one ofthe live resistors constituting theresistor bank 26.

Selector switch 28 controls a relay RLH for disabling the countercircuit in the event that an excess number of count pulses are receivedduring the time interval established by the timing cycle of theelectronic timer as previously described. Relay RLH has a pair ofnormally closed ycontacts which are connected in series in the groundreturn line 31. Selector switch 28 has live possible positions forselecting any one of the output signals S5 to S9, inclusive, and forapplying same to the operating coil of relay RLH. Thus if the circuit isset to accept five pulses selector switch 28 is in its next to theleft-hand position so as to receive signal S5. If a sixth pulse isreceived by the coun-ter the sixth counter stage becomes conductive, andoutput lead S6 delivers positive potential to the operating 4coil ofrelay RLH `which consequently interrupts the circuit and disables thecounter.

Thus it will be seen that if the requisite number of pulses is notreceived fast enough the counter will become disabled upon thevcompletion of the timing cycle of the electronic Itimer associated withswitch 27, before delivering any output signal to the timing mechanismof FIGURE 5. If an excess pulse is received within the permitted timeperiod it is fed back via switch 28 to RLu and again disables thecounter. If the correct number of pulses occurs within the required timeperiod then the output signal produced by counter 21 is applied viaselector switch 29 to the timing mechanism of FIGURE 5.

Each of selector switches 27, 2S and 29 has live possible positions, andthe three switches are mechanically linked together. The five positionsof switch 29 correspond to output leads S4 to S8, inclusive. Thus, thecircuit is set to accept four pulses, switch 29 transmits signal S4 tothe timing mechanism of FIGURE 5. Switch 28 in its correspondingposition is connected to output lead S5 for disabling the counter if anexcess pulse is received. Switch 27 is in the corresponding position andselects a resistance value providing an appropriate timing interval forthe reception of four pulses.

As ywill be explained in connection with FIGURE 5 an output signalgenerated by the counter of FIGURE 3 must be available for apredetermined time period in order to actuate the timing mechanism ofFIGURE 5. In the case where one or more excess pulses are received,therefore, the circuit of FIGURE 3 actually transmits an output pulsevia switch 29, but the time duration of this output signal isinsufficient to turn on the lights since the counter circuit is disabledby the next-received pulse.

Reference is now made to FTGURE 4 where the details of a suitablecounter circuit are illustrated. Thyratron tubes T1, T2, T3, T4 and T5are utilized as the conducting elements of corresponding stages.Referring in detail to the first stage, a load resistor il is coupledbetween the B+ power supply line and the plate of tube T1, while a relaycoil 42 is coupled between the cathode and ground return line 3l. Thegrid of T1 receives bias potential from a negative source Ecc via a gridresistor 43. Positive pulses applied to the input line 2li aretransmitted to the igrid of T1 via a coupling capacitor 44. Resistor 3'5is connected between input line 2li and ground. Inductance coil 42controls the contacts of a relay RL1. Relay RL1 Ihas an upper contactelement 46a, a lower contact element 46c and a middle `Contact elementlieb which selectively engages either the upper element or the lowerelement. The middle contact element 5b normally engages the upperelement 46a when no energizing current is present in coil 42. The uppercontact element 46a is connected to output lead S1, while the middleelement lob is connected to the B|- supply line. Therefore, whenthyratron T1 is not conducting the B+ potential is supplied via outputlead S1 for maintaining capacitor of the electronic timer of FIGURE 3 ina charged condition.

The lowver ycontact element 46c of relay RL1 is coupled through aresistor 41a to the plate of tube T2. The second stage is generallysimilar to the iirst stage but receives energizing potential at itsplate only when the iirst stage is conducting so as to maintain relayRL1 in its operative (as distinguished from its quiescent) condition.Relay RL2 associated with the cathode circuit of tube T2 has only twocontacts, which are normally open but which become closed when T2 isconductive and thereby cause plate potential to be applied via resistor41]; to T2. The third and fourth stages are constructed in the samemanner as the second stage. In the lifth stage an output lead S4 isconnected to the upper end of the plate load resistor 41d. Output leadS4 is therefore energized at the B+ potential when T4 is conducting andits associated relay is closed. 'The remaining stages of the counter,while not illustrated in detail, may be identical to the fifth stage.

Reference is now made to FIGURE 5 illustrating the timing mechanismwhich is selectively yactuated by the circuit of FiGURE 3, and which linturn `controls the airport lights. The input signal from counter 2l isapplied via a time-delay circuit Sti to the operating coil of a relayRL12. rThe purpose of time-delay network 56 is to delay the operation ofrelay RL12 for a suflicient time interval to permit the circuit ofFIGURE 3 to discriminate against any excess pulses vwhich may bereceived. The circuit constants are preferably 'adjusted so that thetime required to operate relay RL12 corresponds approximately to thetime required to receive two successive pulses. For example, if pulsesare received at the rate of one per second, if the circuit is set toaccept live pulses, and if the electronic timer of FIGURE 3 is set at61/2 seconds, it will then require approximately two seconds afterreception of the iifth pulse to actuate relay RL12. A sixth pulsereceived at the same spacing as the previous pulses of the series 'woulddisable counter 2l before relay RL12 could operate. On the #other hand,if the pilot were to transmit the first iive pulses at one-secondintervals and then wait at least two seconds before transmitting thesixth pulse, the result `would be that the landing lights would turn on.It might be said that in this instance the circuit fails to discriminateagainst the sixth or excess pulse, but this result occurs only Where theexcess pulse has a far greater spacing after its predecessor than do theprevious pulses in the series.

`Relay RL12 includes -a `first pair of contacts 55 and a second pair ofcontacts 57. Contacts 55 are connected in series with the operating coilof a relay RL13, the series combination being connected across an A.C.voltage source. Actuation of relay RL12 causes both sets of its contactsto close with the result that a holding relay RL1.,g becomes energized.

A mechanical timer 6@ is utilized which yaccomplishes the primaryfunction of keeping the lights turned on for` a predetermined time.'Ilhe timer 60 is of conventional design and operates as follows. Anindicator needle 61 indicates the amount of time elapsed since themechanism was actuated. A pair of electrical terminals 3 and 5 areprovided, to which an alternating electrical potenti-al is applied foractuating the mechanical timer. A set of relay contacts 6, '7 and S areprovided whose operation is controlled by the timer. Contact element 6normally engages element 7 and is not connected to element 8. When themechanical timer reaches a predetermined setting, relay element 6 movesto an alternate position in which it engages contact element S ratherthan element 7. When the electrical potential applied to terminals 3 and5 is released the element 6 returns to its initial position in which itengages element 7. The timer is Ifurther characterized by the fact thatif prior to reaching its predetermined time setting the potentialapplied to terminals 3 and 5 is released, reapplioation of potential tothe terminals 3 and 5 causes indicator `61 to return to its zeroposition and the timer to run through its complete cycle before relayelement 6 is actuated to its `alternate position.

Terminal 5 of timer titl is connected to one side i8 of the AJC. linewhile terminal 3 is coupled through the contacts of relay RL13 andcontacts 59a, 5-9b or relay R144, to the other side 49 of the A.C. line.The contacts of relay RL13 are normally closed. Therefore, when holdingrelay RL14 `is energized in response to the actuation of relay RL12, theresult is that line potential is applied to terminals 3 and `5 of thetimer 6l) which therefore commences its timing cycle.

In holding relay KLM a pair of contacts 58 are 'connected in series withthe holding coil. The contact elements 53 are also connected in parallelwith contact pair 57 in relay RL12. A thermal timing unit 70 has a pairof normally closed relays which are serially connected between AC.supply line 48 and one of contact elements 58. The other end of theholding coil of relay RL14 is connected through a release switch 71 tothe other A C. supply line 49.

Energization of holding relay RL14 takes place as follows. Actuation ofRL12 causes contacts 57 to close, thus providing a closed circuitbetween contacts 58 of RLM. Since release switch '7l is normally closedthere is a continuous current path pnovided from A.C. line 49 throughthe holding coil of RL14, contacts 58, and the normally closed contactsof timer 70, to the A.C. line 48. The flow of current through theholding coil of RL14 7 closes contact pairs 58 and 59. The closing ofcontacts 58 insures the continued flow of current through the holdingrelay which therefore remains closed.

Resetting or extending the timing period of timer 69 takes place asfollows. If relay RLM is closed the closing of contacts 57 in responseto an input signal has no additional effect upon RLM. Actuation of relayRL12 produces a surge of current through the coil of relay RLlg whichcauses the normally closed contacts of ELI?J to momentarily open. Sincethe applied potential is removed, timer Gil therefore resets itself tothe beginning of its cycle. When the contacts of RL13 return to theirclosed position the line potential is again applied to terminals 3 and5, and timer o@ is again in operation.

Timer 6l) is set for a predetermined time, for example, thirteenminutes. At the end of that interval the movable element 6 disengageselement 7 and engages element 8, thus bringing into operation both a-thermal timer 7d and a asher unit di?. rIhe operation of these lattertwo units Will now be described. Flasher 39 includes an enclosure 8dwithin lwhich a thermal switching element 82 and a heating element 33are serially connected. Switching element S2 closes when the temperaturewithin the enclosure drops to a predetermined lower level, thusestablishing a closed circuit for heating element S3. When thegeneration of heat raises the temperature to a predetermined upperlevel, switching element SZ opens and interrupts the circuit. Current issupplied to the ilasher via a lead 84 connected to A C. line 4S and alead 55 connected to contact 59h of relay RLM.

Flasher `Sli controls a relay RL15 whose actuating coil is connected inkparallel with heating element 83. The contacts of R145 control the lowof current to relay ELN, |which in turn controls the turning on of theairport lights.

Flasher unit Si? and relay RLl are disabled except when holding relayRLM is closed. One of the contacts of RL15 is connected to contact S91)of RLM, while the other is connected to one end 37a of the actuatingcoil of RL16. The other end S717 of the actuating coil is connected tothe A C. line 48.

Movable element `6 of timer 6@ is connected to contact 5911 of RLM,While contact element 7 is connected at A87a to RLlG. The contacts 6* oftimer titl and the contacts of RL15 are therefore effectively inparallel.

During the timing cycle of timer di? element 6 engages contact 7 thusproviding current to RLIG for keeping the airport lights turned on. `Atthe same time the asher 30 and relay RL15 are elfectively shunted out ofthe circuit, although the flasher receives driving power and continuesto operate.

When timer eti completes its cycle movable element 6 disengages contact7 and engages contact 8. Current then for the first time becomessupplied via Contact 8 of timer 60 to the control winding of thermaltimer 7b. Current is supplied to R'Ll only intermittently through thecontacts of RL15 under control of liasher unit Sti, and the lights flashon and ott. When suiicient heat has been generated the normally closedcontacts of timer 70 open, thus `de-energizing holding relay RLM anddisabling the entire circuit.

Thermal timer 7t preferably has an operating period of two minutes,during which the lights are flashed on and olir intermittently. FlasherSi) is preferably selected so that it is on for five seconds and then oifor one second. Thus the lights flash once every six seconds, or twentytimes during the period of control by thermal timer '79.

It will therefore be seen that the timing mechanism of FIGURE normallyturns on the lights for thirteen minutes and then flashes the lights (ina six-second cycle) for two minutes. The lights are then automaticallyturned off. But if the aircraft pilot transmits a new set of turn-onsignals the entire timing cycle again starts from its beginning. This istrue whether the new set of turnon signals are received during the firstthirteen minutes when the lights are burning continuously, or during thelast two minutes while the lights are flashing.

While not specifically illustrated herein the circuit of the presentinvention may advantageously be used for other purposes of airportcontrol. lFor example, the intens-ity of the landing lights may be setat any one of several intensity levels. Also, the wind indicator may beturned to a particular angular position in order to permit the pilot toobserve the Wind velocity in that particular direction. The convenienceof transmission of signals by the pilot, as Well as the extremelyreliable operation of the circuit in distinguishing and responding tothe received pulses, combine to make the apparatus of the presentinvention useful for a variety of different purposes.

Although my invention is fully capable of achieving the results andproviding the advantages hereinbefore mentioned, it is to `be understoodthat it is merely the presently preferred embodiment thereof, and thatwe do not mean to be limited to the details of construction abovedescribed other than as defined in the appended claims.

We claim:

l. Apparatus for turning on the landing lights of an airield in responseto the reception of a radio carrier wave of a predetermined frequencyintermittently transmitted from an aircraft which is about to make anight landing, said apparatus including means for generating a pulse ofa predetermined polarity each time the carrier wave is lturned on, anelectronic counter circuit, means for applying said pulses to saidcounter circuit, output circuit means coupled to said counter circuitfor producing an output signal in response to the last one of apredetermined num- I er of said pulses, time delay means coupled to saidoutput circuit means for receiving said output signal, light controlapparatus coupled to said time delay means for turning on the landing`lights in response to the delayed output signal, means for disablingsaid electronic counter circuit and said output circuit means apredetermined time period after the iirst of said pulses is applied tosaid counter, and additional means for disabling said electronic countercircuit and said output circuit means in response 'to the reception ofan excess pulse by said counter circuit within said predetermined timeperiod.

2. Apparatus as claimed in claim l in which the time delay provided bysaid time delay means is greater than the average time spacing betweensaid pulses when said predetermined number of pulses is applied to saidcounter circuit within said predetermined time period.

3. Airport control apparatus for controlling a predetermined operationin response to the intermittent reception of a predetermined carrierfrequency, said apparatus including means for generating a pulse of #apredetermined polarity each time the reception of the carrier wave isinitiated, an electronic counter circuit, ,means for applying saidpulses to said counter circuit, means for producing an output signal inresponse to the reception by said counter circuit of the last -one of `apredetermined number of said pulses, means for disabling said outputsignal means unless all of said predetermined number of pulses areapplied to said counter within a predetermined time period, andadditional means for disabling said output signal means in response tothe application of an excess pulse to said counter within saidpredetermined time.

4. Apparatus as claimed in claim 3 wherein said pulse generating meansincludes a receiver having an automatic volume control circuit; a relayhaving an operating coil and a pair of normally open contacts, saidoperating coil being coupled to said automatic volume control circuit tobe driven thereby; and circuit means connecting said pair of contacts inseries with the input of said electronic counter circuit; said circuitmeans being operable to pass only positive pulses to said electroniccounter circuit.

5. Apparatus as claimed in claim 4 which further includes a resistorcoupled in parallel with said contacts, and a capacitor coupled inparallel with said contacts.

References Cited in the le of this patent UNITED STATES PATENTS ByrnesOct. 5, 1937 Macalpine Mar. 18, 1941 Mallory July 27, 1943 Houck Sept.14, 1948

3. AIRPORT CONTROL APPARATUS FOR CONTROLLING A PREDETERMINED OPERATIONIN RESPONSE TO THE INTERMITTENT RECEPTION OF A PREDETERMINED CARRIERFREQUENCY, SAID APPARATUS INCLUDING MEANS FOR GENERATING A PULSE OF APREDETERMINED POLARITY EACH TIME THE RECEPTION OF THE CARRIER WAVE ISINITIATED, AN ELECTRONIC COUNTER CIRCUIT, MEANS FOR APPLYING SAID PULSESTO SAID COUNTER CIRCUIT, MEANS FOR PRODUCING AN OUTPUT SIGNAL INRESPONSE TO THE RECEPTION BY SAID COUNTER CIRCUIT OF THE LAST ONE OF APREDETERMINED NUMBER OF SAID PULSES, MEANS FOR DISABLING SAID OUTPUTSIGNAL MEANS UNLESS ALL OF SAID PREDETERMINED NUMBER OF PULSES AREAPPLIED TO SAID COUNTER WITHIN A PREDETERMINED TIME PERIOD, ANDADDITIONAL MEANS FOR DISABLING SAID OUTPUT SIGNAL MEANS IN RESPONSE TOTHE APPLICATION OF AN EXCESS PULSE TO SAID COUNTER WITHIN SAIDPREDETERMINED TIME.